Thermoplastic elastic adhesives

ABSTRACT

Hot melt pressure sensitive adhesive prepared from (a) a rubbery block copolymer containing a rubbery polyisoprene midblock portion and a plurality of crystalline poly(vinylarene) endblocks; (b) a tackifying resin compatible with the midblock portion and (c) poly(alkyloxazoline); and (d) an antioxidant. These adhesives are characterized by exceptional elastic memory, high tensile strength and excellent peel adhesion and, as such, are especially suited for use in elasticized bands for disposable items.

BACKGROUND OF THE INVENTION

Elasticized leg and waist bands have gained increasing popularity in theareas of disposable applications, such as in disposable diapers,incontinent pads and hospital gowns. A commerically viable approach foruse on high speed production equipment was developed by Buell anddisclosed in U.S. Pat. No. 4,081,301. According to this method,continuous bands of elastic, maintained in a stretched position, areglued or heat sealed onto the continuous web from which the disposablediapers are made.

Recognizing that greater efficiency could be achieved if the adhesion ofthe elastic band to the substrate did not require a separate adhesive orheating means, attempts have been made to prepare adhesives whichexhibit the required degree of elasticity without sacrificing thenecessary adhesive and cohesive properties. In U.S. Pat. No. 4,259,220,Bunnelle et al. proposed viscoelastic hot melt pressure sensitivecompositions prepared from a rubbery block copolymer containing arubbery polyisoprene midblock portion and a plurality of crystallinepoly(vinylarene) endblocks; and two different resins, one of which is atackifying resin compatible with the midblock and the other areinforcing resin for the endblock portion of the copolymer.

SUMMARY OF THE INVENTION

The present invention is directed to thermoplastic hot melt adhesivecompositions which function as elastic bands for disposable items andparticularly for leg or waist band closures on disposable diapers. Inparticular, the present invention discloses a hot melt pressuresensitive adhesive prepared from (a) a rubbery block copolymercontaining a rubbery polyisoprene midblock portion and a plurality ofcrystalline poly(vinylarene) endblocks; (b) a tackifying resincompatible with the midblock portion; (c) poly(alkyloxazoline), and (d)an antioxidant.

The resulting adhesives are characterized by exceptional elastic memory,high tensile strength and, in particular, are characterized by excellentpeel adhesion to the polyethylene and nonwoven substrates generally usedfor disposable applications. While not wishing to be bound by theory, itis believed that these properties are achieved from the inherentadhesive strength contributed by the high molecular weightpoly(alkyloxazoline) polymer to the entire hot melt adhesive systemrather than by a mere reinforcement of any portion of the blockcopolymer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The primary component of the adhesive compositions used in the presentinvention are block copolymers having the general configuration:

    A--B--A or A--B--A--B--A--B

wherein the polymer blocks A are non-elastomeric polymer blocks which,as homopolymers have glass transition temperatures above 20° C., whilethe elastomeric polymer blocks B are isoprene. Further, they may belinear or branched. Typical branched structures contain an elastomericportion with at least three branches which can radiate out from acentral hub or can be otherwise coupled together.

The non-elastomeric blocks which make up approximately 17 to 75%, byweight of the block copolymer may comprise homopolymers or copolymers ofvinyl monomers such as vinyl arenes, vinyl pyridines, vinyl halides andvinyl carboxylates, as well as acrylic monomers such as acrylonitrile,ethacrylonitrile, esters of acrylic acids, etc. Monovinyl aromatichydrocarbons include particularly those of the benzene series such asstyrene, vinyl toluene, vinyl ylene, ethyl vinyl benzene as well asdicyclic monovinyl compounds such as vinyl naphthalene and the like.Other non-elastomeric polymer blocks may be derived from alpha olefins,alkylene oxides, acetals, urethanes, etc. Styrene is preferred.

Typical of the rubbery block copolymers useful herein are thepolystyrene-polyisoprene-polystyrene types containing at least 17%styrene which may be prepared using methods taught, for example, in U.S.Pat. Nos. 3,239,478, 3,427,269; 3,700,633; 3,753,936; and 3,932,327.Alternatively, they may be obtained from Shell Chemical Co. for exampleunder the tradename Kraton D1111 or from Phillips under the tradenameSolprene 423. If desired, a portion of these high styrene containingcopolymers can be replaced by those of lower styrene contents as, forexample, Kraton 1107 or Kraton 1117 in order to adjust the melt indecesfor use on various manufacturing equipment. In formulating adhesives foruse herein, the block copolymer should be used in an amount of 35 to75%, preferably 40 to 60%, by weight of the adhesive.

The tackifying resins which are present in the hot melt adhesive usedherein serve to extend the adhesive properties of the block copolymer.As contemplated, the term "tackifying resin" comprises any tackifyingresin compatible with the isoprene midblock and includes: (1)polyterpene resins having a softening point, as determined by ASTMmethod E28 58T, of from about 60° to 140° C. the latter polyterpeneresins generally resulting from the polymerization of terpenehydrocarbons, such as the bicyclic mono-terpene known as pinene in thepresence of Fridel-Crafts catalysts at moderately low temperatures. (2)phenolic-modified terpene resins such, for example, as the resin productresulting from the condensation in an acidic medium, of a bicyclicterpene and a phenol; (3) aliphatic petroleum hydrocarbon resins havinga Ball and Ring softening point of from about 60° to 140° C., the latterresins resulting from the polymerization of monomers consistingprimarily of olefins and diolefins; and (4) hydrogenated copolymers ofalpha-methyl styrene and styrene having a softening point of about 78°to 125° C.

Especially preferred are resins which are polymerized from a stream ofaliphatic petroleum derivatives in the form of dienes and monoolefinshaving 5 or 6 carbon atoms generally in accordance with the teachings ofU.S. Pat. No. 3,577,398. The resulting solid hydrocarbon resin consistsessentially of polymerized structures derived from these aliphaticdienes and mono-olefins of 5 or 6 carbon atoms and since the dienes aremore reactive, at least 40 percent by weight and preferably a majorproporation of said structures are derived from the dienes. In this typeof resin, the dienes are piperylene and/or isoprene. However, in someformulations, the percentage of isoprene is extremely low. In the solidresin of this embodiment the molecular weight may range between about900 and 1300 with the mean number average molecular weight being about1100. This solid resin also has a softening point in the neighborhood of100° C. In one preferred form, i.e., Wingtack 95 offered by GooyearChemical Company, the softening point is 95° C. These tackifying resinsare present in an amount of 10 to 45, preferably 20 to 40%, by weight ofthe adhesive.

While any of the poly (alkyloxazoline) compounds disclosed in U.S. Pat.No. 4,474,928 may be used herein, the most readily available is poly(2-ethyloxazoline), a compound consisting primarily of repeating units(I) which is prepared by the ring opening polymerization of2-ethyl-2-oxazoline (II). ##STR1## The ring-opening polymerization of2-ethyl-2-oxazoline is generally conducted in the presence of a cationicpolymerization catalyst at a reaction temperature of about 0° C.-200° C.Typical catalysts include strong mineral acids, organic sulfonic acidsand their esters, acidic salts such as ammonium sulfate, Lewis acidssuch as aluminum trichloride, stannous tetrachloride, borom triflourideand organic diazoalumfluoroborates, dialkyl sulfates and other likecatalysts. This ring-opening polymerization is further described byTomalla et al. J. Polymer Science, 4,2253 (1966); Bassiri et al. PolymerLetters, 5,871 (1967); Seeliger, Ger. No. 1,206,585; Jones and Roth,U.S. Pat. No. 3,640,909; and Litt et al., U.S. Pat. No. 3,483,141. Thepre-hydrolyzed polymer thereby obtained are linear, N-acylatedpolyethylamines having a molecular structure consisting essentially ofrepeating units (I). These polymers can be used as such or the partiallyhydrolyzed (deacylated) by contact with a strong acid, such as HCl,followed by contact with a base, such as NaOH. Alternatively, thispolymer is available from Dow Chemical Corp. under the name PEOx.

Typically, the poly(alkyloxazoline) has a molecular weight within therange of 1,000 to 1,000,000. In the present invention, it is preferableto use poly(2-ethyloxazoline) having a molecular weight within the rangeof about 50,000 to about 500,000 with the lower molecular weight rangebeing especially preferred. The poly(alkyloxazoline) is used in theadhesive in an amount of 5 to 25%, preferably 10 to 20% by weight.

Antioxidants for use herein include high molecular weight hinderedphenols and multifunctional phenols such as sulfur andphosphorous-containing phenols. Hindered phenols are well known to thoseskilled in the art and may be characterized as phenolic compounds whichalso contain bulky radicals in close proximity to the phenolic hydroxylgroup thereof. In particular, tertiary butyl groups generally aresubstituted onto the benzene ring in at least one of the ortho positionsrelative to the phenolic hydroxy group. The presence of these radicalsin the vicinity of the hydroxyl group serves to retard its stretchingfrequency and correspondingly, its reactivity. This steric hindrancethus provides the phenolic compound with its stabilizing properties.Representative hindered phenols include:1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzylbenziene;pentaerythrityltetrakis-3(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate,n-octadecyl-3(3,5-di-tert-butyl-4-hydroxyphenyl)proprionate,4,4'-methylenebls (2,6-di-tert-butylphenol); 2,2'-methylenebis(4-methyl-6-tert-butylphenol); 4,4'-thiobis(6-tert-butyl-o-cresol);2,6-di-tert-butylphenol; 6-(4-hydroxyphenoxy)-2,4-bis(n-octylthio)-1,3,5 triazine;2,4,6-tris(4-hydroxy-3,5,-di-tert-butylphenoxy)-1,3,5-triazine;di-n-octadecyl-3,5-di-tert-butyl-4-hydroxy-benzylphosphonate;2-(n-octylthio)ethyl)3,5-di-tert-butyl-4-hyroxy-benzoate, and sorbitolhexa-[3-(,5-di-tert-butyl-4-hydroxy-phenyl)propionate.] The antioxidantand is generally used at levels of 0.2 to 2% by weight.

The performance of these antioxidants may be further enhanced byutilizing, in conjunction therewith (1) synergists such, for example, asthiodipropionate esters and phosphites; and (2) chelating agents andmetal deactivators such, for example, as ethylenediamine tetraaceticacid.

Optional additives may be incorporated in minor amounts, generally lessthan 3% by weight, into the hot melt compositions in order to modifycertain properties thereof. Among these additives may be includedcolorants such as tianium dioxide; fillers such as talc and clay, etc.,hydrocarbon process oils; etc.

These hot melt adhesive compositions may be formulated using techniquesknown in the art. An exemplary procedure involve placing the tackifyingresin, poly (alkyloxazoline), stabilizer and any optional additiveswhose presence may be desired in a jacketed mixing kettle, preferably ina jacketed heavy duty mixer of the Baker Perkins or Day type, which isequipped with rotors and thereupon raising the temperature to a range offrom about 250° to 350° F., the precise temperature utilized dependingon the melting point of the particular tackifying resin. When the resinhas melted, stirring is initated and the block polymer, is added, theaddition of the polymer being extended over a prolonged period in orderto avoid the formation of lumps. Mixing and heating are continued untila smooth, homogeneous mass is obtained.

In the examples that follow, the adhesive prepared were subjected to thetests described below:

Tensile Strength:

Tensile strength values for the elastic of this invention are used as ameasure of cohesive strength. A sample of known thickness is elongatedat room temperature using an Instron and the tensile strength at 100%elongation is recorded. Samples having tensile strengths of at leastabout 35 psi at 100% elongation provide adequate cohesive strength.

Dead Load Deformation:

The term "dead load deformation" or "dead load creep" refers to ameasurement of "cold flow" or permanent deformation at one or more fixedtest temperatures, e.g., 23° C. or 25° C., 40° or 41° C. and 49° or 50°C. A sample of known length is suspended vertically in a chambermaintained at the test temperature and a mass is attached to the lower(free) end of the sample. The sample is cut to a size such that theforce per unit area is 1500 g/cm². After approximately 3 hours at thetest temperature, the sample is removed, the weight is detached, and thesample is allowed to relax under the influence of its own inherentelastomeric forces. The length of the relaxed sample (L₂) is compared tothe original length (L₁) and the "dead load creep" (permanentdeformation) is determined according to the formula (L₂ -L₁)/L₁ ×100%.Values of less than 20 % deformation are considered adequate.

T-peel:

Peel adhesion values are determined 24 hours after formation of the bondbetween cold-rolled steel plates and the self-adhering elastic. The bondis formed at room temperature by applying a 0.5"×20 mil ×3"elastic/adhesive strip onto the steel, backing it with masking tape, andsealing at room temperature using 20 psi pressure for 2 seconds. Thebond is tested using an Instron. Values of at least about 4 pounds perlinear inch indicate acceptable tack levels for the more stringent legbanding applications, however lower levels are acceptable for lowertension applications such as waist banding.

Polyethylene film and Non Woven peel:

A 20 mil film of adhesive is made using a Carver press at 350° F. under8,000 lbs pressure with a residence time of approximately 15 sec. Then3×1/2" samples are cut and backed with masking tape. The samples arebonded to non-woven or 2 mil polyethylene film backed with tape using 2passes with a 41/2 lb. roller. The bond is peeled immediately using anInstron at 12"/min. Results are recorded in grams per linear inch.

The following examples will further illustrate the embodiments of theinvention. In these examples all parts given are by weight and alltemperatures in degrees Celsius unless otherwise indicated.

EXAMPLE 1

Adhesives were prepared in a Sigma mixer heated to 170° C. by blendinguntil homogeneous the components in the amounts shown in Table I. Theadhesives were then tested as described above with the results alsoshown in Table 1.

                  TABLE I                                                         ______________________________________                                                   Adhesive                                                           Component    1        2        3      4                                       ______________________________________                                        Kraton 1111  55       55       55     55                                      PEOx         15       15       15     15                                      XAS-10874.01 (1)                                                              Wingtack 95 (2)                                                                            30       --       --     --                                      Nirez V-2150 (3)                                                                           --       30       --     --                                      Regalrez 1078 (4)                                                                          --       --       30     --                                      Escorez 5300 (5)                                                                           --       --       --     30                                      Dead load      6%      20%      15%    10%                                    Deformation                                                                   Tensile Strength @                                                                         55 psi   55 psi   43 psi 39 psi                                  100%                                                                          T-peel       5.5/L.I. 3.0/L.I. 5.5/L.I.                                                                             5.0/L.I.                                Polyethylene peel                                                                          770      *        *      *                                       Nonwoven peel                                                                              600      *        *      *                                       ______________________________________                                         (1) Molecular weight approximately 50,000                                     (2) A C.sub.5 terpene resin from Goodyear Chemical Company                    (3) A terpenephenolic tackifying resin from Reichhold Chemical                (4) A hydrogenated styrenealpha-methyl styrene resin from Hercules            Chemical                                                                      (5) A hydrogenated dicyclopentadiene resin from Exxon Chemical                *Not tested                                                              

EXAMPLE 2

Other adhesives were prepared using different amounts of the componentsas well as blends of the rubber copolymers. Compositions and testsresults are shown in Table II.

                  TABLE II                                                        ______________________________________                                                 Adhesive                                                             Component  5       6        7      8     9                                    ______________________________________                                        Kraton D-1111                                                                            50      40       40     40    55                                   Kraton D-1117                                                                            --      10       --     --    --                                   Kraton D-1107                                                                            --      --       10     --    --                                   PeOx XAS   17.5      17.5     17.5 20    15                                   10874.01                                                                      Wingtack 95                                                                              32.5      32.5     32.5 40    20                                   Tensile    48      44       53     39    79                                   Strength @ 100%                                                               Dead Load  10%       5%       5%    10%   15%                                 Deformation                                                                   T-Peel     7/L.I.  6.5/L.I. 5.5/L.I.                                                                             13/L.I.                                                                             3/L.I.                               ______________________________________                                    

In a similar manner, other thermoplastic hot melt adhesives may beprepared using other poly(alkyloxazolines) including, for example,poly(2-ethyloxazolines) of different molecular weights as well as thosepoly(alkyloxazolines) where the alkyl group is hydrogen, methyl, propyl,pentyl, cycloheyl, dodecyl, octadecyl, as well as the varioushalogenated or ethylenically unsaturated derivatives thereof, such aspoly(2-trichloromethyl-2-oxazoline), poly(2-isopropenyl-2-oxazoline),etc.

Now that the preferred embodiments of the present invention have beendescribed in detail, various modifications and improvements thereon willbecome readily apparent to those skilled in the art. Accordingly, thespirit and scope of the present invention is to be limited only by theappended claims, and not by the foregoing disclosure.

We claim:
 1. A thermoplastic hot melt adhesive compositioncomprising:(a) from 35 to 75% by weight of a rubbery block copolymercontaining a rubbery polyisoprene midblock portion and a plurality ofcrystalline poly (vinylarene) endblocks wherein the polyvinylareneportion constitutes from 17 to 75% by weight of the copolymers; (b) from10 to 45% by weight of a tackifying resin selected from the groupconsisting of polyterpene resins having a softening point, as determinedby ASTM method E28 58T, of from about 60° to 140° C.; phenolic-modifiedterpene resins; aliphatic petrol hydrocarbons having a Ball and Ringsoftening point of from about 60° to 140° C.; and hydrogeneratedcopolymers of styrene and alpha-methyl strene having a softening pointof from 78° to 125° C.; (c) from 5 to 25% by weight ofpoly(alkyloxazoline); and (d) from 0.2 to 2% by weight of anantioxidant.
 2. The adhesive composition of claim 1 wherein the blockcopolymer is a polystyrene-polyisoprene-polystyrene copolymer.
 3. Theadhesive composition of claim 2 wherein the polystyrene portioncomprises at least 22% by weight of the block copolymer.
 4. The adhesivecomposition of claim 2 wherein the polystyrene portion comprisesapproximately 22% by weight of the block copolymer.
 5. The adhesivecomposition of claim 1 wherein the poly(alkyloxazoline) ispoly(2-ethyloxazoline).
 6. The adhesive composition of claim 3 whereinthe poly(alkyloxazoline) is poly(2-ethyloxazoline).
 7. The adhesivecomposition of claim 1 wherein the block copolymer is present in anamount of 40 to 60% by weight of the adhesive.
 8. The adhesivecompositions of claim 1 wherein the tackifying resin is present in anamount of 20 to 40% by weight of the adhesive.
 9. The adhesivecomposition of claim 1 wherein the tackifying resin is a syntheticpolyterpene resin having a softening point of 60° to 140° C.
 10. Theadhesive composition of claim 1 wherein the poly(alkyloxazoline) ispresent in an amount of 10 to 20% by weight of the adhesive.
 11. Theadhesive composition of claim 5 wherein the poly(2-ethyloxazoline) has amolecular weight of about 50,000.